In optical transmission systems, optical pulses travelling along a transmission medium are affected by dispersion, which causes individual pulses to be distorted by neighbouring pulses in time, a phenomenon known as inter-symbol interference (ISI). Consequently, decisions about a transmitted pulse, which are based upon the received but distorted version of that pulse, will be inaccurate, leading to a high bit error rate (BER).
In order to combat inter-symbol interference in optical signals, various techniques have been proposed. For example, a purely optical solution provides for a dispersion compensation fiber (DCF) at intervals of several kilometers along the transmission path. A DCF is a specially doped fiber which re-aligns the pulses travelling therealong in time. However, DCFs are not only expensive but also ineffective for long-haul and dense wavelength-division multiplexed (DWDM) systems.
Other proposed techniques have borrowed from the field of electrical signal equalization. These include linear tapped delay structures which are directly applied to the electrical version of the received optical signal following opto-electronic conversion. While such techniques may improve system performance, they tend to do so only to a limited extent since they can only compensate for linear components of the ISI. Conventional approaches fail to take into account that the opto-electronic conversion process in the receiver leads to non-linearities in the ISI and also to non-Gaussianity of the noise statistics, neither of which can be compensated for successfully through the use of a conventional equalizer.
Thus, there is a need in the industry to provide an improved system and method for detecting received optical symbols, especially in the presence of inter-symbol interference.